Liquid crystal display devices have been widely used conventionally as thin and light-weight color display devices. Among such color liquid crystal display devices, most commonly used are transmissive liquid crystal display devices which employ a back light source. The transmissive liquid crystal display devices have been used in an increasingly wider variety of field for various uses.
What contrasts to the transmissive liquid crystal display devices are reflective liquid crystal display devices which employ other display modes, whereby reflected light of a light source (natural light or surrounding light) is used for display. The reflective liquid crystal display devices therefore utilize a light source instead of a back light and do not require a back light, thus having such features as reducing power for the back light and saving a space or weight thereof.
That is, power consumption of the display device can be reduced as a whole, which permits the use of smaller batteries, making the reflective liquid crystal display devices suitable for equipment which is required to be thin and light-weight. Further, given the same size or weight of the equipment, the reflective liquid crystal display devices allows the use of larger batteries, making it possible to greatly increase the operation time.
Further, the reflective liquid crystal display devices also have advantages over other display devices in view of contrast ratio characteristics of the display. That is, in self-emitting display devices such as a CRT, a significant reduction in contrast ratio is incurred under day light outside. Such a significant reduction in contrast ratio occurs also in transmissive liquid crystal display devices, that low reflection films are coated on, when the intensity of the surrounding light is much larger than display light, as in the case under direct sun light. On the other hand, the reflective liquid crystal display devices can obtain display light which is proportional to the quantity of the surrounding light, and can avoid a reduction in contrast ratio, and therefore are suitable particularly for portable information terminals, digital video cameras, or portable video cameras, etc., which are often used outside.
Despite such promising applications, there has been no reflective color liquid crystal display device which meets the demand for practical applications. This is chiefly due to the fact that conventional reflective color liquid crystal display devices were insufficient in terms of reflectance contrast ratio, full-color display, high-definition display, and their ability to display moving images.
The following describes conventional reflective liquid crystal display devices in more detail. Currently, the reflective liquid crystal display devices which are widely used employ a pair of or a single polarizer. The operation modes of these liquid crystal display devices include a twist nematic mode (“TN mode” hereinafter) which performs display by controlling optical rotatory power of the liquid crystal layer by an electric field, a birefringence mode (“ECB mode” hereinafter) which performs display by controlling birefringence of the liquid crystal layer by an electric field, and a mix mode, which is a combination of the TN mode and the ECB mode.
Meanwhile, there have been known reflective liquid crystal display devices which do not employ a polarizer. Guest-Host-type liquid crystal elements, which incorporate a dye in liquid crystal, have been developed for this mode, which, however, had the problem of low reliability due to the addition of the dichroic dye, and the problem of low contrast ratio which is posed by the low dichroic ratio of the dye. This deficiency in contrast ratio in particular results in a significant reduction in color purity in color display using a color filter. Therefore, such reflective liquid crystal display devices which lack a contrast ratio need to be combined with a color filter having high color purity. The reflective liquid crystal display devices therefore have the problem of low brightness when the high color purity color filter is used, which spoils the advantage of high brightness of the mode which omits the polarizer.
In order to overcome the foregoing problems, there has been developed a liquid crystal display element of a mode which employs a polymer-dispersed-type liquid crystal or a cholesteric liquid crystal, which is intended for bright and high-contrast ratio display without using a polarizer or a dye. These modes take advantage of the characteristic of the liquid crystal layer which is optically switched between a transmissive state and a scattering state, or between a transmissive state and a reflective state, by controlling an applied voltage to the liquid crystal layer. Further, no polarizer is required in these modes and the efficiency of using light can be improved.
Further, from the perspective of evaluation on color fidelity, a desirable white state can be expected in these modes compared with the TN mode or ECB mode, because the wavelength dependency is low and the problem of absorption profile of the polarizer itself, i.e., the problem of the polarizer absorbing blue light and the light transmitting through the polarizer is rendered yellow, is not posed.
Such a mode is disclosed, for example, in Japanese Unexamined Patent Publication No. 186816/1991 (Tokukaihei 3-186816) (publication date: Aug. 14, 1991). In the liquid crystal display device in this publication, a polymer-dispersed-type liquid crystal is disposed on a black substrate, wherein a white/black state is performed by the white state, which is rendered by the scattering state of the polymer-dispersed-type liquid crystal which appears murky under no applied voltage, and by the black state, which is rendered by the transmissive state of the polymer-dispersed-type liquid crystal through which the underlying black substrate becomes visible under applied voltage.
U.S. Pat. No. 3,905,682 (publication date: Sep. 16, 1975) discloses a liquid crystal device having a light modulating layer using a light-scattering-type liquid crystal, and a retro-reflector. Japanese Unexamined Patent Publication No. 105998/1979 (Tokukaisho 54-105998) (publication date: Aug. 20, 1979) discloses a reflective liquid crystal display device including a light modulating layer using a light-scattering-type liquid crystal or a Guest-Host-type liquid crystal, louvers, and a retro-reflector. Further, U.S. Pat. No. 5,182,663 (publication date: Jan. 26, 1993) discloses a liquid crystal device including a light modulating layer using a light-scattering-type liquid crystal, and a corner cube array.
However, in the liquid crystal display device of the foregoing publication No. 3-186816, only the backward scattered light from the polymer-dispersed-type liquid crystal contributes to reflectance of the white state in the white state, and the forward scattered light is absorbed entirely by the black substrate, and the actual efficiency of utilizing light suffers greatly.
In the liquid crystal display device disclosed in U.S. Pat. No. 3,905,682, a black state is realized when the liquid crystal layer is in a transmissive state. A display quality of the black state by retro-reflection is dependent on retro-reflectivity, and is strongly influenced by the size of the smallest unit structure of the retro-reflector. However, this patent U.S. Pat. No. 3,905,682 does not teach a mechanism for realizing a black state or a size of the smallest unit structure of the retro-reflector.
Further, the retro-reflector disclosed in the embodiment of this patent is a retro-reflector which is realized by a corner cube array or an array of tiny spheres, neither of which, however, possess sufficient retro-reflectivity, and a desirable black state cannot be obtained. Further, this patent is silent as to a detailed structure of a retro-reflector with sufficient retro-reflectivity. Further, there is a problem of poor display quality of a black state when the display is viewed from a direction inclined with respect to a direction normal to the display surface.
In the reflective liquid crystal display device disclosed in the foregoing publication No. 54-105998, louvers having an absorbing site are disposed on the front side of the retro-reflector on the side of the viewer, and since the retro-reflector is covered with the louvers with respect to light rays incident on the liquid crystal display device from the side of the viewer, all the incident light is absorbed at the absorbing site of the louvers to realize a desirable black state, and the light rays which are incident on the liquid crystal display device from the side of the light source directly reach the retro-reflector through the louvers.
However, this publication is also silent as to the size of the smallest unit structure of the retro-reflector, and a mechanism for realizing a black state, and, while it solves the problem of the U.S. Pat. No. 3,905,682, brightness in white state suffers because the area occupied with the absorbing site of the louvers is too large.
Further, none of the foregoing conventional arrangements consider a relation between a pitch of the smallest unit structures of the retro-reflector and a pitch of the color filters. Therefore, when the color filters are provided in the foregoing arrangements, rays of incident light and outgoing light pass through different color filters, which results in reduction in luminance and chromaticity due to mixed colors.
The foregoing problems are also common in reflective display devices in general, other than the liquid crystal display devices.